In the past few years, a great deal of work has been done to develop rare earth fiber amplifiers for use in optical communications systems. At present, the greatest success in this has been achieved with Er-doped silica fibers for use in amplifying 1.5 um optical signals. In fact, optical amplifiers based on Er-doped fibers are now available for long haul communications systems to replace electronic regenerators in which optical signals are converted to electrical signals before they are amplified. In contrast to this situation at 1.5 um, similar success has not been achieved in fabricating fiber amplifiers for use in amplifying 1.3 um optical signals which occur in the wavelength region at which most installed optical systems operate. At present, there are two main rare earth candidates for use in fabricating 1.3 um fiber amplifiers; neodymium and praseodymium.
Neodymium has a transition which produces fluorescence near 1.3 um between the 4F3/2 level and the 4I13/2 level. However, excited state absorption ("ESA") at about 1.3 um degrades amplifier performance. In particular, in a silica fiber host, 1.3 um ESA prohibits gains at wavelengths shorter than about 1.36 um. Although changing the host to ZBLAN glass causes a slight shift of the gain spectrum to shorter wavelengths, gains at 1.31 um are still low. In addition, the branching ratio for emission in Neodymium at 1.06 um and 0.9 um limits emission at 1.3 um to 10% since the more efficient emission at the shorter wavelength depletes the 3F3/2 level and, thereby, reduces amplification at 1.3 um.
A further problem occurs in fabricating fiber amplifiers for many laser transitions due to quenching by the host. Quenching occurs because of the presence of a level, below an upper laser level, which is relatively close in energy to the upper laser level. Unfortunately, this situation applies to a large number of potential laser transitions. As a result of this, i.e., the nature of optical transitions involved in producing light at a given wavelength, the selection of a glass host is crucial in fabricating fiber light amplifier devices.
In light of the above, a praseodymium doped fluoride fiber is currently viewed as the most promising candidate for fabricating a 1.3 um amplifier due to the absence of an absorbing transition competing with a laser transition between the 1G4 upper level and the 3H5 lower level in Pr.sup.3+ in the desired wavelength region of 1.26 um-1.31 um. In fact, amplification has been demonstrated in Pr-doped fluorozirconate glass fibers at 1.31 um. Despite this, however, the selection of a host glass is still critical in this case because of the close proximity of lower lying levels to the 1G4 upper laser level. As one can see from the energy level diagram of Pr.sup.3+ shown in FIG. 1, the energy gap between the 1G4 upper laser level and the 3F4 lower lying level manifold is approximately 3000 cm.sup.-1. Further, it is known that non-radiative transitions from the 1G4 level to the 3F4 level manifold will be decreased in a host whose highest energy phonon has relatively small energy. For example, in a fused silica host, the highest energy phonon has an energy of approximately 1100 cm.sup.-1. Since only three such phonons need be emitted to bridge the gap between the 1G4 level and the 3F4 level manifold, non-radiative transitions between these levels are very rapid. As a result, there is no measurable emission at 1.3 um. However, it has been demonstrated in the art that, in a heavy metal fluoride glass host, non-radiative transition probabilities are substantially reduced relative to those occurring in a fused silica host--the highest energy phonon for a heavy metal fluoride glass host is approximately 500 cm.sup.-1. Thus, in a heavy metal fluoride glass host, approximately six phonons need to be emitted to bridge the gap between the 1G4 level and the 3F4 level manifold. Because of the larger number of phonons required for this non-radiative transition, the transition probability is less than that for a fused silica glass. The resulting difference in transition probability makes it possible to obtain laser radiation from the transition between the 1G4 level and the 3H5 level. Nevertheless, even with such an improvement associated with the use of a heavy metal fluoride host, the quantum efficiency for fluorescence for this case is still only 3%. This low quantum efficiency is evidenced by a relatively short fluorescent lifetime of approximately 0.1 ms for emission from the 1G4 level. In addition, a calculation from Judd-Ofelt Parameters gives a radiative lifetime from the 1G4 level of approximately 3 ms. This supports the conclusion of a quantum efficiency for fluorescence of only 3%.
In light of the above, there is a need for a glass composition having a low energy phonon spectrum which can serve as a host for active materials for use in fabricating light sources such as fiber laser oscillators, light amplifiers, and superluminescent sources. In particular, there is a need for such a glass composition which can be used to fabricate light sources wherein the active materials comprise rare earth materials. In further particular, there is a need for such a glass composition which can be utilized to fabricate light sources wherein the active materials comprise rare earth materials for producing radiation substantially at 1.3 um.